The present invention relates to mirrors for use in various applications, including, for example, free space optical communication systems.
Wireless communication systems involve the transmission and reception of wireless signals. To date, wireless communication systems have been mostly designed to operate in the radio frequency (xe2x80x9cRFxe2x80x9d) spectrum. The information-carrying capacity of a wireless communication system is determined by the frequency (or wavelength) of the wireless signal. More particularly, higher frequency (or shorter wavelength) wireless signals provide greater information-carrying capacity than lower frequency signals. With the demand for greater information-carrying capacity increasing, it has become apparent that the RF spectrum may shortly become less attractive for some (e.g., urban or metro) wireless communication applications.
Various alternatives for increasing the information-carrying capacity of wireless communication systems have been explored. One such alternative currently being considered is using the optical spectrum for signals in place of the RF spectrum. Given their relatively shorter wavelengths, optical signals support higher data transmission speedsxe2x80x94upwards of 10 gigabits per secondxe2x80x94than RF wireless signals. With these data transmission speeds, free space optical communication systems offer greater information-carrying capacity than wireless systems designed to operate in the RF spectrum.
Typically, free space optical communication systems include at least one transmitting unit and at least one receiving unit. The transmitting and receiving units are each mounted on a platform located, for example, on top of a building structure, to provide an unobstructed path for the transmission and reception of free space optical signals. Each unit in a free space optical communication system includes at least one mirror for optically linking (e.g., coupling) the transmitting and receiving units together. More particularly, the mirror within the transmitting unit conveys a free space optical signal from a light source to the mirror in the receiving unit. In turn, the mirror in the receiving unit conveys the received free space optical signal to a photodetector.
Free space optical signals propagate in a more directional manner (e.g., point to point) than RF wireless signals. Consequently, precise alignment between the mirrors of the transmitting and receiving units is required to insure the complete reception of a free space optical signal. Achieving such precise alignment, however, is dependent on environmental conditions, since each unit is mounted on the top of a building structure, for example. When exposed to a wide range of temperatures (e.g., from xe2x88x9240xc2x0 F. to 120xc2x0 F.), the precise shape (e.g., the curvature) of a typical, inexpensively manufactured mirror is compromised. Thus, if such a mirror were to be used in a free space optical communication system, the precision .of the alignment may be compromised because the coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) of the materials employed in manufacturing the mirrors and mountings, for example, may not sufficiently match.
In view of these shortcomings, mirrors used in free space optical communication system have, to date, been made by known process techniques associated with the manufacture of high grade, optical quality components. One such process technique has included machining the surface of a bulk metal substrate. Mirrors manufactured using these process techniques maintain their precise shape (e.g., structural integrity), and thusly, their alignment, in the face of exposure to a wide range of temperatures (e.g., from xe2x88x9240xc2x0 F. to 120xc2x0 F.). Machining the surface of a bulk metal substrate, however, is a time consuming procedure, adding considerable expense in the manufacture of the mirror.
We have invented a relatively less time consuming and inexpensive method of making a high grade, optical quality reflective element, such as a mirror. More particularly, we have invented a reflective element able to maintain its precise shape if exposed to a wide range of temperatures (e.g., from xe2x88x9240xc2x0 F. to 120xc2x0 F.). In one aspect of the present invention, our reflective element comprises a reflective layer formed on a thixotropic metal alloy substrate. For the purposes of the present invention, thixotropic metal alloy, at a particular temperature below melting and above solid state, has a slurry (e.g., semi-solid or plastic like) form. Illustratively, the thixotropic metal alloy substrate is advantageously formed using a casting, molding or injection-molding step, and may also include, for example, shaping, cooling or solidifying the thixotropic metal alloy within a mold, for example.
In another aspect of the present invention,. our reflective element comprises a reflective layer formed on an injection-molded substrate. Here, the injection-molded substrate advantageously comprises a thixotropic metal alloy, which is shaped, cooled and/or solidified, for example, within the injection-mold. One process technique for forming the thixotropic-based metal substrate is Thixomolding(copyright), wherein a thixotropic metal alloy, comprising magnesium to date, is injection-molded while in a slurry form.
We have recognized that a mirror having a thixotropic metal substrate is capable of maintaining its precise shape over a considerably wider range of temperatures than a typical, inexpensively manufactured mirror. In one example of the present invention, our mirror is formed by shaping, cooling or solidifying a thixotropic metal alloy slurry injected into a mold, and subsequently forming a reflective layer thereon. Advantageously, this approach is less time consuming and considerably less expensive than the machining techniques associated with the manufacture of precision optical quality components.
We have also recognized that the smoothness of the surface of a cast, molded or injection-molded thixotropic metal alloy mirror may be insufficient for certain applications. More particularly, we have recognized that injection-molding a thixotropic metal alloy to form the substrate may create microscopic irregularities greater than 1 xcexcm in size. Microscopic irregularities include, for example, crevices, divots, holes or bumps along the surface of substrate.
In the present invention, the smoothness of the surface of the cast, molded or injection-molded thixotropic metal alloy substrate may be improved by incorporating a polymeric interface layer between the substrate and the reflective layer. The polymeric interface layer acts as a quasi-conformal coating. For the purposes of the present invention, quasi-conformal means conforming to the curvature of the substrate, if the substrate is curved, while providing a smooth surface for the reflective layer: that does not conform crevices, divots, holes or bumps along the surface of substrate greater than a particular size. The size of the microscopic irregularities allowable along the surface of substrate is a function of the wavelength of the electromagnetic energy to be reflected by the reflective element. More particularly, the surface of the substrate of our mirror should be free of microscopic irregularities within xe2x80x9c+xe2x80x9d or xe2x80x9cxe2x88x92xe2x80x9d one-quarter (xc2xc) of the signals"" wavelength (xcex). Given the wavelengths of optical signals in the broadband communications spectrum range between 1.3 xcexcm and 1.6 xcexcm, the surface of the substrate of our mirror should be minimally free of microscopic irregularities of greater than 0.5 xcexcm. However, it will be apparent that for wavelengths in the upper bands of broadband communications spectrum range, the surface of the substrate of our mirror should be free of microscopic irregularities of greater than 0.3 xcexcm or, perhaps even less. The polymeric interface layer comprises a thermoset material, such as an epoxy resin, for example. By incorporating the polymeric interface layer, the surface of the substrate is free of microscopic irregularities greater than +/xe2x88x92(xc2xc) of the optical signals"" wavelength (xcex).